The mechanism of the partial oxidation of methane to synthesis gas (CO+H-2) was investigated over a nickel-on-alumina catalyst. The pulse reaction experiments combining XPS and TPR investigations were used to verify the active sites and selective O species present on the working catalyst. It was found that both, reduced Ni and NiOx species are present on the catalyst surface during the reaction. Unlike the highly dispersed surface NiAl2O4 species on the fresh catalyst which are reducible at ca. 800 degrees C, these NiOx species present on the working catalyst can be reduced at 520-560 degrees C, they are thus assigned to adsorbed O atoms. The comparisons of the pulse reactions of CH4, CH4/O-2 (2:1) and CH4/CO2(1 : 1) indicate that the reduced Ni sites play a role in dissociating methane into surface C atoms and H-2, while the direct formation of CO occurs via reacting surface C atoms with adsorbed O atoms present as NiOx. We propose that the primary surface reaction of the partial oxidation of methane over Ni/Al2O3 proceeds via a CH4 pyrolysis followed by the reduction of NiOx by the surface C atoms (arising from CH4 dissociation), which is reoxidized by incorporation of O-2 into the catalyst.